In the Southern Ocean, a carbon-dioxide mystery comes clear

February 3, 2016

Researchers have found that bottom waters of the Southern Ocean had very low levels of oxygen during the last ice age, indicating high uptake of carbon. Here, dissolved Southern Ocean bottom-water oxygen in modern times. Brighter colors indicate more oxygen; dots show sites where researchers sampled sediments to measure past oxygen levels. Credit: Jaccard et al., Nature 2016

Twenty thousand years ago, when humans were still nomadic hunters and gatherers, low concentrations of carbon dioxide in the atmosphere allowed the earth to fall into the grip of an ice age. But despite decades of research, the reasons why levels of the greenhouse gas were so low then have been difficult to piece together.

New research, published today in the leading journal Nature, shows that a big part of the answer lies at the bottom of the world. Sediment samples from the seafloor, more than 3 kilometers beneath the ocean surface near Antarctica, support a long-standing hypothesis that more carbon dioxide was dissolved in the deep Southern Ocean at times when levels in the atmosphere were low.

Among other things, the study shows that during the ice age, the deep Southern Ocean carried much smaller amounts of oxygen than today. This indicates that photosynthetic algae, or phytoplankton, were taking up large amounts of carbon dioxide near the surface. As dead algae sank to the depths, they were consumed by other microbes, which used up the oxygen there in the process. The scientists found chemical fingerprints of the oxygen level by measuring trace metals in the sediments.

The evidence "is a long-sought smoking gun that there was increased deep ocean carbon storage when the atmospheric CO2 was lower," said Sam Jaccard of the University of Bern, Switzerland, the study's lead author.

Coauthor Robert Anderson, a geochemist at Columbia University's Lamont-Doherty Earth Observatory, said the study "finally provides the long-sought direct evidence that extra carbon was trapped in the deep sea by the buildup of decaying organic matter from above." He added, "It's also clear that the buildup and release of CO2 stored in the deep ocean during the ice age was driven by what was happening in the ocean around Antarctica."

The study also shows that variations in carbon-dioxide storage in the Southern Ocean were probably behind a series of natural "wobbles" in atmospheric levels of about 20 parts per million that took place over thousands of years. The study suggests that the wobbles were probably caused by changes in the amount of iron-rich dust, which fertilizes phytoplankton, being blown from land onto the ocean surface. Levels may also have been influenced by varying amounts of carbon being released from the deep ocean as ocean currents changed, said the authors.

The study may hold powerful lessons for today. While the natural 20-part-per million wobbles took thousands of years to happen, carbon dioxide levels have risen that much in just the last nine years, due to human emissions. Levels are now about 400 parts per million, versus about 280 in the early 1800s. "The current rate of emissions is just so fast compared to the natural variations that it's hard to compare," said study coauthor Eric Galbraith of the Autonomous University of Barcelona. "We are entering climate territory for which we don't have a good geological analog."

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9 comments

The premise of papers like this is that the Earth is a self-contained system which exhibits no significant history related to its surroundings. Yet, modern astronomy is plainly a witness to the catastrophic nature of space. We see that transient events like rogue brown dwarfs are really quite common, but our specialized approach to geological science fails to take such observations into account. The modern approach to understanding the past suffers from its obvious specialist fragmentation.

You see the problem in comments online as well: attempts to interest specialists in relevant issues in related disciplines commonly evokes a complaint that they'd prefer to keep the discussion "on topic".

It's honestly self-destructive behavior. Science struggles to predict and explain transient events, but that does not make them any less real. The world is not in a box, folks.

promile

Time lag makes sense, since higher temperatures mean lower solubility of CO₂, as with any gas. Oceans have huge storing capacity, so there's more CO₂ dissolved in the ocean as there is CO₂ in the atmosphere. If you increase the temperature, whatever the primary reason may be, the CO₂ levels will rise.

Twenty thousand years ago, when humans were still nomadic hunters and gatherers, low concentrations of carbon dioxide in the atmosphere allowed the earth to fall into the grip of an ice age

Uh, no, not exactly.

Growth of the ice sheets reached their maximum positions 26,500 years ago. Deglaciation commenced in the Northern Hemisphere approximately 19,000 years ago, and in Antarctica approximately 14,500 years ago.

It's honestly self-destructive behavior. Science struggles to predict and explain transient events, but that does not make them any less real. The world is not in a box, folks.

Which catastrophes are you on about? K-T impact? Permian mass extinction? All these type of events are well studied, and leave tell tale imprints of various kinds. So what are you proposing to replace the well understood Milankovich cycles with, and what is the evidence for it?

Well, the contemporary global warming may be followed with escaping of carbon dioxide into an atmosphere again

This is *only* the case where dissolved concentration of anything is close to the asymptote of saturation @ particular temperature range, where's *any* indication we are close to that *anywhere* at all ?

If you can find such comparative data for sea-water then U 'might' have a case, then you can do the math re solubility constants and integrate over the oceans isotherms, can you appreciate vague claims implying a slight increase will result in CO2 release is woefully incomplete, disingenuous and false, you have to be complete re the chemistry/Physics of solubility.

Accepting climate disaster as a reliable forecast it no longer makes sense to continue funding science except climate science since all scientific knowledge will be erased along with the human race. Redirect the funding of all other sciences immediately and send the useless non climate scientists to the waiter/dishwasher/pot economy that is now America's economic growth sector.

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